In practical terms, i.e. when buying some phones for actual money; can we safely conclude that frequency graphs like the ones on Headroom are adequate indicators of sound quality?

A "neutral" frequency response, is the first thing that people ask when looking for a headphone.But it's not enough to warrant good sound quality.In other hand, you'd see people rejecting some headphones models , just by looking at the frequency responses.It's the case of the expensive denon d7100, there's an important bass emphasis, if you look at he graph.

All made using professional head and torso simulator equipment costing thousands of dollar, yet the graphs differ. There's simply no consensus on measurement and calibration standards at the moment.

Your links are broken and I disagree. Look at the raw data. Differences at the very low end are due to seal problems (see different raw curves at innerfidelity), difference at high frequencies are due to placement.

Your links are broken and I disagree. Look at the raw data. Differences at the very low end are due to seal problems (see different raw curves at innerfidelity), difference at high frequencies are due to placement.

If you'll look at the graphs you'll see its more than just the low and highs.